Compressors are typically used in production processes across the oil and gas, energy and chemicals industries, to increase the pressure of a gas. Highly specific to the respective processes they are designed to support, they tend to be custom-manufactured, expensive and difficult to replace.
Compressors operate with very fast dynamics and even the slightest disturbance to what is designed to be a steady state deems them highly vulnerable to compressor surge. Simply put, this is the point at which a centrifugal compressor cannot add enough energy to overcome the system’s backpressure.
Affecting compressed air systems, compressor surge is a form of aerodynamic instability that occurs in axial and centrifugal compressors. It takes the form of an event whereby violent air flow oscillates in the axial direction of a compressor, and the energy contained within the gas becomes either equal or larger than the energy expended by the rotating parts of the compressor. Because the energy accumulated in the compressed gas becomes greater than the energy levels in the rotating impeller, a backdraft effect occurs, and the gas expands rapidly in an abnormal direction back through the compressor.
Centrifugal compressor surge can be incredibly dangerous and damaging for both the compressor, and the entire machine. It puts substantial strain on the compressor’s parts, including bearings, seals and the impeller itself, therefore the energy swell caused by a compressor surge can lead to the unit malfunctioning, including rapid flow reversals and changes in axial thrust and high vibration which could necessitate equipment replacement. The vibration caused by the surging can severely damage the motor compressor coupling and also the baseplate. Finally, and most importantly, there is a significant risk to those individuals working near the compressor, as a surge can result in a process becoming hazardous, particularly if process gas is released into the surrounding atmosphere.
Breaking the cycle
The biggest problem with compressor surge, however, is the fact that if it isn’t addressed, it can become a cyclical event. This is because when the flow reverses, the energy and pressure is relieved and drops down, and the compressor can compress fluid which in turn increases pressure. Once the gas energy volume dissipates by flowing back into the suction of the compressor, the operation of the compressor should return to normal, but if preventative measures are not taken, the compressor will proceed with a further surge. If that surge is not stemmed, it becomes a cyclical event, with frequency dependent on the design of the compressor and on ancillary parts.
Common causes of compressor surge
Compressor surge is very much a system phenomenon, therefore pinpointing an exact cause can be a challenge. Typically, it occurs as a result of the infrastructure of the piping system, which is comprised of pipes, valves and volumes, along with a compressor. The highly vulnerable nature of the system design makes surge an inherent risk.
Inaccuracies or misinterpretations of key terms used when describing the different types of compressor surge, can exacerbate this risk. For example, violent surge refers to the intermittent nature of the flow reversal which creates large forces capable of damaging compressor bearings, seals, and other rotating elements. However mild surge, while an instability, doesn’t have the same full reversal of flow. The salient point here is that the same compressor can develop mild surge or violent surge, so it’s important to understand the geometry of the infrastructure, both upstream and downstream of the compressor, in order to ascertain the likelihood of occurrence.
There are many different ways to control compressor surge, but choosing the best approach isn’t always straightforward. Prevention is always best, but a strategy focused on prevention requires a combination of in-built control systems and digital condition monitoring capabilities.
Prevention is better than cure
Understanding compressor surge, and the specific conditions which can cause it, is fundamental in mitigating its occurrence and impact.
To combat compressor surge, specialised control systems are available which can be designed and implemented into processes. These seek to maintain a steady fluid flow to the compressor, which keeps the unit from surging.
In parallel, adopting a predictive maintenance system such as Senseye PdM, which captures data via sensors and other means, to facilitate live condition monitoring. Vibration monitoring for example, whether using basic MEMS or high-precision sophisticated accelerometers, can provide deep insights into sensitivity and flag potential surge scenarios. Similarly, changes in pressure and flow can be monitored, with the platform set up to identify specific patterns or anomalies which point to surge risk.
While compressor surge is a substantial concern for those operating machinery which uses compressed air systems, concerns can be substantially mitigated through applying dedicated and comprehensive measures along the lines of those outlined above. Protecting and securing investment in these expensive systems, while prioritising safety is crucial, and for those operating such equipment without a strategy to mitigate compressor surge, the time to act is now.